Modified SIR values for fast power control

ABSTRACT

The present invention relates to a receiver comprising a fast power control unit, said fast power control unit being arranged to continuously control a quality measure of a radio channel. The receiver is characterized in that the quality measure is a modified Signal to Interference plus noise ratio (SIR) in which the influence from self interference has been removed. The invention further relates to a method for continuously controlling a quality measure of a radio channel, wherein a modified Signal to Interference plus noise ratio (SIR) is continuously determined in which the influence from self interference has been removed.

TECHNICAL FIELD OF THE INVENTION

The present invention relates to a receiver comprising a fast powercontrol unit, said fast power control unit being arranged tocontinuously control a quality measure of the radio channel.

The present invention also relates to a method for continuouslycontrolling a quality measure of a radio channel.

DESCRIPTION OF RELATED ART

The fast power control loop in a CDMA system has two purposes. First, itis used to maintain the Carrier to Interference ratio (C/I) needed for agiven quality of service. The quality is measured in Block Error Rate(BLER), Bit Error Rate (BER), throughput or some other measure. Tooptimise the capacity of the system it is important that the Carrier toInterference ratio (C/I) is neither too small nor too large. Secondly,the fast power control is used to combat fast fading.

However, in the fast power control loop it is common to measure theSignal to Interference plus noise Ratio (SIR) instead of the Carrier toInterference ratio (C/I) and use the Signal to Interference plus noiseRatio (SIR) as an indication of the Carrier to Interference ratio (C/I).The relationship between SIR and C/I depends on the fading profile ofthe channel, bit rate and power settings of the transmission. The fastpower control can however operate without knowledge of the relationshipbetween SIR and C/I and thus without knowledge of the C/I. Instead, thefast power control controls an estimated SIR of some known pilots oreasily detected control symbols against a desired SIR target. The SIRtarget is adapted in a slower control loop, denoted outer power control.The outer power control involves increasing or decreasing the SIR targetin response to a measured quality of service.

A problem with power control addressed herein, is the problem with selfinterference. For low Carrier to Interference ratios and for fadingprofiles with few rays, the self interference is negligible in relationto the noise while for high Carrier to Interference ratios anddispersive channels the self interference is dominating. In thefirst-mentioned case with low Carrier to Interference ratios and fadingprofiles the relationship between the transmitting power and SIR isalmost linear. In the latter case on the other hand with high Carrier toInterference ratios and dispersive channels, the relationship betweentransmitting power and the SIR is highly non-linear and the SIR valueconverges towards an asymptotic value with increasing transmittingpower. This may cause the power control loop to break down and powerrushes to occur. The consequence is that full throughput can not bereached for high bit rate services and highly dispersive radio channels.

US 2005/0201447 relates to a method and apparatus for estimating signalimpairment correlations for one or more received signals of interestusing a model-based technique, wherein the model is adapted in responseto recurring measurements of signal impairment correlations that can bemade on frequent basis. Thereby, even rapidly changing signal impairmentcorrelations can be tracked. In detail, received signal impairmentcorrelations are determined for use in generating RAKE combiningweights.

SUMMARY

One object of the present invention is to provide a way of adapting theSIR so that it will be highly dependent on the transmitted power alsofor high Carrier to Interference ratios and dispersive channels.

In accordance with one embodiment of the present invention this has beensolved by means of a receiver comprising a fast power control unit, saidfast power control unit being arranged to continuously update a qualitymeasure of a radio channel. The receiver is characterized in the qualitymeasure is a modified Signal to Interference plus noise ratio (SIR) inwhich the influence from self interference has been removed.

As the self interference destroys the SIR estimate as a good controlvariable for high Carrier to Interference ratios and dispersivechannels, removing the influence from self interference will result instable fast power control.

The fast power control unit is arranged to compare the determinedquality measure with a predetermined quality value and to, based on saidcomparison broadcast a message to a transmitter for amending thetransmitting power.

In accordance with one embodiment of the invention, wherein the powersettings of the transmitted codes is not known the fast power controlunit comprises a processing unit comprising a parametric model of theradio channel determined at least based on known, demodulated pilots.The model comprises a self interference impairment term R_(ISI) scaledby a first fitting parameter A and a noise impairment term R_(n) scaledby a second fitting parameter B. The processing unit is arranged toadapt values of the first and second fitting parameters A, B to fit themodel to measured received pilots for example using a Least Squaremethod. The model of the radio channel is for example described by theequation R_(m)=A·R_(ISI)+B·R_(n) The self interference covariance matrixR_(ISI) of the equation can be determined based on at least receiveddemodulated pilots and path delays in the radio channel. The processingunit is arranged to determine the modified SIR as a relation between asignal power or the like, the second parameter B and the noiseimpairment term R_(n).

The first and second parameters A and B are by definition non-negative.However, the estimates of the parameters can turn out to be negativeanyway. The likelihood for the estimated first parameter A, to benegative increases when self interference is low and the true valueclose to zero. In order to improve the calculations the processing unitis therefore in accordance with one embodiment of the invention arrangedto determine when the fitted first parameter A decreases below a presetvalue, to thereupon set the first parameter A to zero and re-estimatethe value of the second parameter B and to use the re-estimated secondparameter B in determining the modified SIR.

In order to further improve the estimate of at least the secondparameter B the processing unit is arranged to filter the possiblyre-estimated second parameter B with a Minimum Mean Squared Error (MMSE)based smoothing method and to use the filtered second parameter B indetermining the modified SIR. In an extended embodiment also the firstparameter A is filtered with the Minimum Mean Squared Error (MMSE) basedsmoothing method.

If a limited number of pilot symbols are available, the estimate can bequite noisy. Accordingly, in accordance with one embodiment of theinvention is the variance reduced by applying post-filtering in a filterhaving low-pass characteristics.

In accordance with one embodiment of the invention the fast powercontrol unit is arranged to operate in interaction with a RAKE receiver.The RAKE receiver is then arranged to feed known pilots from therespective finger outputs of the RAKE to the fast power control unit. Inorder to increase the robustness of the determination of the modifiedSignal to Interference plus noise ratio (SIR), the RAKE comprises inaccordance with one embodiment of the invention at least one fingeroutput for each fading path in the radio channel. As the number of RAKEfingers used in a RAKE combiner for the received data is not as criticalas the number of RAKE fingers used by the fast power control unit, thenumber of finger outputs of the RAKE receiver fed to the fast powercontrol unit is larger than the number of finger outputs fed to a RAKEcombiner for data processing.

In an alternative embodiment of the present invention, wherein the powersettings of transmitted codes are known, a processing unit arranged tocalculate the self interference comprises a power estimation unitarranged to estimate a signal power or the like and a noise power or thelike of the pilots or a code in which the pilots are incorporated fromeach RAKE finger output, and to estimate the total signal power or thelike over all codes for each finger output using the signal power or thelike on the pilots or the code in which the pilots are incorporated andknown power settings of the transmitted codes. The processing unitfurther comprises a self interference estimating unit arranged toestimate the self interference for each finger output as thecontribution of the total signal power or the like over all codes andall finger outputs except the finger output for which the selfinterference is estimated.

In order to decrease the variance of the self interference, theprocessing unit comprises in accordance with one embodiment of theinvention a post-filtering unit arranged to filter the self interferenceestimated in the self interference estimating unit by means of a filterhaving low-pass characteristics.

The processing unit comprises also a total interference estimating unitarranged to estimate the total interference for each finger output. Inorder to decrease the variance of the total interference, the processingunit can comprise a post-filtering unit arranged to filter the totalinterference estimated in the total interference estimating unit bymeans of a filter having low-pass characteristics. A SIR estimation unitis arranged to estimate the modified SIR for example as a ratio betweenan estimated signal power and a difference between the possiblypost-filtered total interference and the possibly post-filtered selfinterference.

In accordance with one embodiment of the invention, the receiver is aCDMA, for example a WCDMA receiver.

The present invention relates further to a wireless communicationterminal for use in a wireless communication network comprising areceiver in accordance with the above.

The present invention relates further to a radio base station for use ina wireless communication network comprising a receiver in accordancewith the above.

The present invention also relates to a method for continuously updatinga quality measure of a radio channel. The method is characterized inthat it comprises continuously determining a modified Signal toInterference plus noise ratio (SIR) in which the influence from selfinterference has been removed.

In accordance with one embodiment of the invention the continuousdetermination of the modified Signal to Interference plus noise ratio(SIR) comprises the steps of

-   -   determining a self interference impairment term R_(ISI) scaled        by a first fitting parameter A,    -   determining a noise impairment term R_(n) scaled by a second        fitting parameter B,    -   adapt the first and second fitting parameters A, B to fit a        model of the radio channel to measured received pilots, and    -   determining the modified SIR as a relation between a signal        power or the like, the second parameter B and the noise        impairment term R_(n).

Further features of these embodiments are described in the dependentclaims 23-28.

In accordance with an alternative embodiment of the present inventionestimation of the self interference involves

-   -   estimating a signal power or the like and a noise power or the        like of pilots or a code in which the pilots are incorporated        from each finger output of a RAKE receiver,    -   estimating the total signal power or the like over all codes for        each finger output using the signal power or the like on the        pilots or the code in which the pilots are incorporated and        known power settings of the transmitted codes, and    -   estimating the self interference for each finger output as the        contribution of the total signal power or the like over all        codes and all finger outputs except the finger output for which        the self interference is estimated. The total interference is        then estimated and the difference between the total interference        and the self interference is used in determination of the        modified SIR.

The present invention also relates to a method for fast power control,characterized by the steps of continuously updating a quality measure ofa radio channel in accordance with any of the method embodimentsdescribed above, comparing the determined quality measure with apredetermined quality value, and based on said comparison broadcasting amessage to a transmitter for amending the transmitting power.

The present invention also relates to a computer program productcomprising computer readable code means which, when run on a computersystem performs the method in accordance with any of the methodembodiments described above.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a block diagram over a receiving unit in accordance withone example of the present invention.

FIG. 2 shows a block diagram over a fast power control unit of thesystem in FIG. 1.

FIG. 3 shows a block diagram over a processing unit of the fast powercontrol unit in FIG. 2 in accordance with one example of the presentinvention.

FIG. 4 shows a block diagram over a processing unit of the fast powercontrol unit in FIG. 2 in accordance with an alternative example of thepresent invention.

FIG. 5 shows a flow chart of the processing of the processing unit inaccordance with the example as described in FIG. 3.

FIG. 6 shows a flow chart of the processing of the processing unit inaccordance with the example as described in FIG. 4.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

In FIG. 1 a receiving unit 1 comprises one antenna 2 arranged to receivea signal carried through the air, an A/D converter 3, a receiver filter4, a pilot demodulator 5 a, a data demodulator 5 b, a decoder 6 and afast power control unit 7.

The signal received by the antenna 2 is for example a radio signal.Further, the signal is composed by a number L of rays {r_(k)}_(k=1) ^(L)8 a, 8 b, 8 c transmitted through the air, each ray 8 a, 8 b, 8 creaching the antenna 2 from an individual fading path. In the example ofFIG. 1, three rays are shown. Each ray has its own attenuation and phaseshift described by the channel. In FIG. 1, the rays are described bytheir power {γ_(k)}_(k=1) ^(L) and channel response {g(t)_(k)}_(k=1)^(L).

The signal is transmitted by means of CDMA (Code Division MultipleAccess) transmission. Accordingly, N orthogonal spreading codes{C_(i)}_(i=0) ^(N−1) are transmitted, wherein one or several codes areused for user data and one or several codes are used for control data.The codes have power settings {β_(i) ² }_(i=0) ^(N−1), describing thepower distribution between the codes, and spreading factors{SF_(i)}_(i=0) ^(N−1). Further, it is assumed that for example C₀contains known pilots.

The total received signal power at the antenna 2 is then given by theequation

$C = {\left( {\sum\limits_{i = 0}^{N - 1}\beta_{i}^{2}} \right) \cdot {\left( {\sum\limits_{k = 1}^{L}\gamma_{k}} \right).}}$

In an alternative embodiment (not shown), the receiving unit comprisestwo or more antennas.

The signal received by the antenna 2 is processed in an analogue radioreceiver and transmitted to the A/D converter 3 arranged to convert thereceived analogue signal to a time discrete signal suitable for furtherprocessing. The converted signal is fed to the receiver filter 4. Thefunction of the receiver filter 4 will not be described in detailherein.

The pilot and data demodulators 5 a, 5 b, are arranged to demodulate thereceived signal, whereby the influence on the signal resulting from themultipath propagation is removed. This will not be described in detailherein. However, in brief, a conventional RAKE receiver can be used forthis purpose. RAKE receivers are well known in the communication artsand find widespread use in Code Division Multiple Access (CDMA) systems,such as in IS-95, IS-2000 (cdma2000) and Wideband CDMA (WCDMA) wirelesscommunication networks. In general, multiple, parallel receiver fingersare used to receive multiple signal images in a received multipathsignal. By coherently combining the finger outputs in a RAKE combiner,the RAKE receiver can use multipath reception to improve the Signal-toNoise Ratio of the received multipath signal.

In an alternative example, a so called parametric generalized RAKE isused in the demodulators 5 a, 5 b. The parametric generalized RAKE isarranged to calculate RAKE combining weights and SIR estimates byproviding a model of received signal impairment correlations for thereceived signal comprising an interference impairment term scaled by afirst fitting parameter A and a noise impairment term scaled by a secondfitting parameter B, measuring received signal impairment correlationsat each of one or more successive time instants, and, at each timeinstant, fitting the model to measured received signal impairmentcorrelations by adapting instantaneous values of the first and secondfitting parameters A, B, and maintaining the model by updating the firstand second parameters A. B, based on the instantaneous values calculatedfor them at each instant. The impairment correlations are used tocalculate alternative/new weights for the RAKE combiner.

The pilot demodulator 5 a is arranged to demodulate the pilots, asdiscussed above for example contained in the code C₀. The pilotdemodulator 5 a then provides a first set of demodulated pilots y₁ alongwith information related to a path delay τ₁ for each fading path havingat least a minimum quality and received by the antenna 2.

There exist searcher methods well known in the art for obtaining thepath delays τ₁ for the fading paths, and accordingly this will not bedescribed in detail. Other time delays in addition to the path delaysmay be used. The first set of demodulated pilots y₁ along withassociated time delays τ₁ are fed to the data demodulator 5 b. The datademodulator 5 b is arranged to demodulate the data with the aid of thepilots y₁ with associated path delays τ₁ using RAKE fingerscorresponding to the fingers used in providing the pilots y₁ withassociated path delays τ₁. The demodulated data signal is then fed tothe decoder 6, wherein the data signal is decoded.

The pilot demodulator 5 a also provides a second set of demodulatedpilots y₂ along with information related to a path delay τ₂ for eachfading path having at least a minimum quality and received by theantenna 2 (this minimum quality can be different than the minimumquality required for providing the demodulated pilots y₁ to the datademodulation unit 5 b). The second set of demodulated pilots y₂ alongwith associated path delays τ₂ are fed to the fast power control unit 7.

In one example, the pilot demodulator 5 a is arranged to provide thesame set of demodulated pilots y₁, y₂ to the data demodulator 5 a and tothe fast power control unit 7. In another example, the first set ofdemodulated pilots y₁ is a subset of the second subset of demodulatedpilots y₂. In yet another example, different RAKE:s are used forproviding the first and second set of demodulated pilots.

In FIG. 2, the fast power control unit 7 comprises a processing unit 9arranged to frequently update a quality measure of the signal incomingto the antenna 2, a comparing unit 10 arranged to compare the qualitymeasure with a quality measure target value stored in a memory 11 and aradio transmitter 12 arranged to broadcast a message in dependence ofthe result of the comparison. Characteristically, the quality measure isupdated 1000 times per second or more for example 1500 times per second.Conventionally, the processing unit 9 of the fast power control unit 7is arranged to calculate an estimated Signal to Interference plus noiseRatio (SIR) of the known pilots y₂ or easily detected control symbolsand to feed the calculated SIR value to the comparing unit 10 arrangedto compare the calculated SIR with a set SIR target. The SIR target isset for example based on the number of fading paths received by theantenna: the setting of the SIR target will not be described in detailherein. If the comparison shows that the SIR lies outside allowableranges for the SIR target, the radio transmitter 12 is arranged tobroadcast a message informing that the power of the transmission has tobe altered. This is called fast power control. In using the fast powercontrol, the fast fading of the channels g can be compensated for sothat it is secured that a desired Block Error Rate (BLER), Bit ErrorRate (BER), throughput or some other measure of the signal quality isachieved.

In accordance with the present invention the processing unit 9 isarranged to calculate an estimated, modified SIR value. The modified SIRvalue is then used for comparison with a SIR target.

In FIG. 3, the execution steps executed by the processing unit 9 of thefast power control unit 7 for calculating the estimated, modified SIRvalue are shown. In a first processing step, a unit 13 for estimation ofcovariance matrix is arranged to estimate a covariance matrix R_(m) fromthe received demodulated pilots y₂. The demodulated pilots y₂ arederived from each finger output of the RAKE receiver, as describedabove.

In one example, the estimation of the covariance matrix R_(m) involvesestimating the channel response for each finger of the RAKE receiver. Wedenote the channel response experienced by the receiver, i.e. filteredby receiver and transmitter filters, the net channel response. The netchannel response could for example be estimated as

$c_{k} = {\frac{1}{p}{\sum\limits_{i = 0}^{p - 1}{y_{2,k}(i)}}}$

The covariance matrix R_(m) is then estimated per slot from pdemodulated pilot symbols y₂. The k:th row and l:th column of R_(m) iscalculated as

${R_{m}\left( {k,l} \right)} = {\frac{1}{p - 1}{\sum\limits_{i = 0}^{p - 1}{\left( {{y_{2,k}(i)} - c_{k}} \right) \cdot \left( {{y_{2,l}(i)} - c_{l}} \right)^{*}}}}$where y_(2,k)(i) and y_(2,l)(i) are the i:th demodulated pilot symbolsand c_(k) and c_(l) are the net channel responses, for finger k and l,respectively, and wherein * denotes complex conjugation.

In a second processing step, a unit 14 for estimation of IntersymbolInterference and noise covariance matrices is arranged to calculate saidIntersymbol Interference covariance matrix R_(ISI) and said noisecovariance matrix R_(n). The self-interference component of thecovariance between two fingers with delays d₁ and d₂ is calculated as

${R_{ISI}\left( {d_{1},d_{2}} \right)} = {\sum\limits_{l}{\sum\limits_{q}{{g_{l} \cdot g_{q}^{*}}{\sum\limits_{{m = {- \infty}},{m \neq 0}}^{\infty}{{R_{p}\left( {d_{1} - {mT}_{c} - \tau_{2,l}} \right)} \cdot {R_{p}^{*}\left( {d_{2} - {mT}_{c} - \tau_{2,q}} \right)}}}}}}$wherein the summation over l runs over all paths originating from thesame antenna as the finger with delay d₁, while the summation over qruns over all paths originating from the same antenna as the finger withdelay d₂:wherein τ_(2,l), are the path delays for the paths originating from thesame antenna as the finger with delay d₁ and τ_(2,q) are the path delaysfor the paths originating from the same antenna as the finger with delayd₂; the path delays are in-signals to the second processing step and arefor example calculated using known searcher methods, as discussed above,wherein g₁ are the medium channel responses for the paths originatingfrom the same antenna as the finger with delay d₁ and g_(q) are themedium channel responses for the paths originating from the same antennaas the finger with delay d₂; the medium channel responses g_(l), g_(q)are estimated from the net channel estimates c by removing the influenceof the receiver and transmitter filters. This constitutes normal stepsto a person skilled in the art, see for example US patent application US2005/0201447.

The noise component of the covariance between two fingers with delays d₁and d₂ is calculated as

${R_{n}\left( {d_{1},d_{2}} \right)} = \left\{ \begin{matrix}{R_{p}\left( {d_{1} - d_{2}} \right)} & {{if}\mspace{14mu} d_{1}\mspace{14mu}{and}\mspace{14mu} d_{2}\mspace{14mu}{belong}\mspace{14mu}{to}\mspace{14mu}{the}\mspace{14mu}{same}\mspace{14mu}{antenna}} \\0 & {{otherwise}.}\end{matrix} \right.$

Now, a model of the covariance matrix can be denotedR _(m) =A·R _(ISI) +B·R _(n),wherein R_(m), R_(ISI) and R_(n) have been estimated in the first andsecond processing steps.

The parameters A and B in the model above are estimated in a thirdprocessing step by a unit 15 for parameter estimation. The estimation ofthe parameters A and B will not be described more in detail herein. Aperson skilled in the art would be able to solve the equation systemabove using computer aided methods. For example, a Least Square methodcan be used in determining the parameters A and B.

The parameters A and B are by definition non-negative. However, duringsome circumstances, the estimates of the parameters A and B provided bythe parameter estimation unit using for example the least square method,may be negative values. The likelihood for the estimated parameter A tobe negative is increased in environments with low self interference,wherein the A value is close to zero.

The estimate for B is in one example further improved by a thresholdingand reestimation unit 16 in a fourth processing step. The fourthprocessing step includes the following thresholding and re-estimationprocedure.

If α<Threshold then set A=0 and re-estimate B as specified below,wherein Threshold is a preset design parameter designed so as tooptimize the performance of the algorithm.

In order to re-estimate B, the matrix B·R_(n) is fitted to thecovariance matrix R_(m). The diagonal elements of these matrixes give aset of real equations. The off-diagonal elements, if used, give a set ofcomplex equations which can be treated as two sets of equations, one setrepresenting the real part of the equations and the other setrepresenting the imaginary part of the equations.

For example, suppose there are two fingers and

${R_{m} = \begin{bmatrix}m_{11} & m_{12} \\m_{12}^{*} & m_{22}\end{bmatrix}},{R_{n} = \begin{bmatrix}r_{11} & r_{12} \\r_{12}^{*} & r_{22}\end{bmatrix}}$

This gives the following four fitting equationsm ₁₁ =Br ₁₁  (1)m ₂₂ =Br ₂₂  (2)Re{m ₁₂ }=BRe{r ₁₂}  (3)Im{m ₁₂ }=BIm{r ₁₂}  (4)

Standard least-squares approaches can be used to find B. Typically, theimaginary part of r₁₂ is zero. In this case, it may be advisable to dropequation (4). Further, for widely spaced RAKE fingers, the real part ofr₁₂ may be zero or at least very small. In this case, it may beadvisable to drop equation (3) as well.

If the parameter B determined by the parameter estimation unit 15 couldbe <0 then B=0, in which case the thresholding and re-estimation unit 16is by-passed. If the parameter B determined by the thresholding andre-estimation unit 16 in one example is <0 then B is set to B=0.

In one example, the parameter B determined by the thresholding andre-estimation unit 16, or if such unit is not present or by-passed,determined by the parameter estimation unit 15, is fed to a filter 17arranged to filter the parameter B. The filter 17 is used for furtherimproving the estimates provided by the parameter estimation unit 15 orthresholding and re-estimation unit 16 in order to provide an improvedestimate B. The filter is for example arranged to work in accordancewith a Minimum Mean Squared Error (MMSE) based smoothing method known inthe art.

The quality of the interference estimate provided by the parameter Bfrom the unit for parameter estimation 15 or thresholding andre-estimation unit 16 or the parameter B from the filter 17 depends onthe number of pilots y₂ available. With a limited number of pilots, theinterference estimate can be quite noisy. The variance can be reduced byapplying filtering in a post-filtering unit 18.

The filter of the post-filtering unit 18 is for example a low-passfilter. For example, the low-pass filter can be denoted{circumflex over (B)} _(m) =λ{circumflex over (B)} _(m−1)+(1−λ) B _(m)where B _(m) is the parameter B fed from the filter 17 for slot m,{circumflex over (B)}_(m) is the output from the post-filtering unit 18for slot m and λ is the forgetting factor of the IIR filter.

The low-pass filtering or the like in the post filtering unit 18 willreduce the variance of the interference estimate, which will improve SIRestimation. Another benefit is that filtering will slow down the fastpower control loop, if there is remaining self interference due to theinstantaneous fading profile. This will reduce variations in the signalpower when the fast power control loop is close to becoming unstable.

The improved and post-filtered estimate {circumflex over (B)} is fed toa unit 19 for calculating a modified SIR value. The modified SIR valuewith self-interference removed is in one embodiment of the calculationunit calculated asmodified SIR=β₀ ²(1/{circumflex over (B)})c ^(H) R _(n) ⁻¹ cwherein c are net channel coefficient estimates (proportional to themedium channel response estimates g using the transformation R_(p) basedon the known cumulative response of transmitter and receiver chip pulseshaping filters, as described above) and H is the Hermitian conjugate,wherein β₀ ² represents the power setting of the code (this term may beomitted depending on how the net channel coefficients are estimated),for example C₀, in which the processed pilots y₂ are incorporated andwherein R_(n) is the noise covariance matrix calculated by the unit 14for estimation of ISI and noise covariance matrices, as described above.

In one simple embodiment, it is for computational reasons assumed thatthe off-diagonal elements of R_(n) are small. Then, the inverse of R_(n)can be approximated by a diagonal matrix whose elements are thereciprocals of the diagonal elements of R_(n). Thus, in accordance withthis simple embodiment of the invention, the modified SIR value iscalculated as

${{Modified}\mspace{14mu}{SIR}} = {\left( {1/\hat{B}} \right){\sum\limits_{k = 1}^{L}\frac{S_{k}}{R_{n}\left( {k,k} \right)}}}$wherein S_(k) is some estimate of the signal power on the k:th path,based for example on the pilot symbols.

The modified SIR value calculated by the unit 19 for calculating amodified SIR value is fed to the comparing unit 10 for comparison withthe preset value, as described in relation to FIG. 2.

When estimating the self interference, or Intersymbol Interference,using the arrangement as described above, Rake fingers should be placedon all significant rays in the radio channel. If there are rays in theradio channel that are not covered by Rake fingers, these rays will giveadditional interference and the additional interference from these rayscould not be identified as self interference, since the signals fromthese rays are not known in the receiver.

The additional interference will put an upper limit of the estimatedSIR, SIR_(limit). The actual limit will depend on power settings and theinstantaneous fading profile of the user. If there is too much power inthe rays not covered by Rake fingers, the SIR limit could become so lowthat there is a risk of occurring power rushes. Thus, it is importantthat enough Rake fingers are available in the receiver for the radiochannels that might occur.

Accordingly, having enough Rake fingers is crucial when demodulating thepilot symbols used for SIR estimation. It is not as important whendemodulating the data symbols, even though self interference is aproblem also here. Due to fading, the power of the rays in the radiochannel will vary. For proper estimation of the modified SIR value, itmust be possible to remove the self interference, or IntersymbolInterference, during all time. Otherwise, there is a risk that themodified SIR value is limited too low, which would result in powerrushes. On the other hand, for the data reception it is sufficient thatthe demodulated signal is good enough most of the time, in order toprovide good data throughput.

Therefore, in one example of the invention, more Rake fingers are usedfor SIR estimation than for data demodulation. This is important ifthere are limitations in the hardware or processing power, so that thenumber of Rake fingers for data demodulation cannot be increased.

The above described processing in the processing unit 9 in order toprovide the modified SIR value does not rely on knowing the powersettings of all transmitted codes for the user. Now, an embodiment withan alternative processing unit 20 is described, in relation to FIG. 4,which can be used if the power settings {β_(i) ²}_(i=0) ^(N−1) of thetransmitted codes are known. In this alternative embodiment, the powersettings {β_(i) ²}_(i=0) ^(N−1) of the transmitted code along with theinformation about the spreading factor SF₀ are stored in a memory 22 ofthe processing unit 20. Inputs to the processing unit 20 in accordancewith this alternative embodiment are the demodulated pilots y₂ derivedfrom each finger output of the RAKE receiver, as described in relationto the embodiment described in relation to FIG. 3. The processing unit20 is arranged to

-   -   estimate the signal and noise power for each finger k on the        code in which the pilot symbols y₂ are incorporated, e.g. C₀,    -   estimate the total signal power over all codes for each finger        k, using the signal power on the code C₀ and the power settings        of the transmitted codes,    -   estimate the self interference for signal power for each finger        k as the contribution of the total signal power over all codes        and all fingers except the finger k,    -   compensate the interference estimates used when estimating SIR        by removing the self interference.

An example of the calculations executed by the processing unit 20 inthis alternative embodiment is given below. The signal powers S_(k) areestimated in a power estimation unit 21 on the code C₀ using pilotsymbols for each Rake finger k. The total interference powers I_(k) areestimated in a total interference estimating unit 24 on the code C₀using pilot symbols for each Rake finger k.

The self interference power is estimated in a self interference unit 23as

$I_{k,{self}} = {\frac{\left( {\sum\limits_{i = 0}^{N - 1}\beta_{i}^{2}} \right) \cdot \left( {\sum\limits_{{m = 1},{m \neq k}}^{L}\frac{S_{m}}{\beta_{0}^{2}}} \right)}{{SF}_{0}}.}$

The modified SIR is estimated in a unit 27 for estimating a modified SIRas

${SIR} = {\sum\limits_{k = 1}^{L}\frac{S_{k}}{I_{k} - I_{k,{self}}}}$

If the denominator is lower than a preset value, or negative, a maximumvalue SIR_(max) for the modified SIR value is chosen instead. Themodified SIR value provided in accordance with this embodiment is usedby the fast power control unit 7, as described above.

One advantage of this alternative embodiment is that it is very simpleand requires few computational steps. A drawback is that the currentpower settings {β_(i) ²}_(i=0) ^(N−1) of the user must be known. Thiscould be solved by using blind detection β_(i) factors.

For e.g. Enhanced Uplink in WCDMA, the power settings can be obtained bydecoding the E-DPCCH control channel. This will introduce a delay in theinterference estimate, since the control channel cannot be decoded untila complete transport block is received. The interference estimate fromone transport block is used in the modified SIR estimation whenreceiving the next transport block. This assumes that blind detection isnot used.

Also in this embodiment, it is important that a sufficient number ofRake fingers are available for demodulating the pilot symbols.Otherwise, not all self interference will be removed, which could limitthe estimated modified SIR, as described in relation to FIG. 3.

Further, in order to reduce variance in the estimated interference,filtering of the interference estimates can be applied equivalent to thefiltering function of the previously described post filtering unit 18.However, in this case, it is preferred to apply filtering to either theself interference estimate I_(k,self) the total interference estimateI_(k), or both.

Accordingly, the estimated total interference is in one example filteredin a post filtering unit 25 in accordance with the following equation

I_(k,m)=λI_(k,m−1)+(1−λ)I_(k,m) for slot m, where λ is the forgettingfactor of the IIR filter.

The estimated self interference is in one example filtered in apost-filtering unit 26 in accordance with the following equation

I_(k,self,m)=λI_(k,self,m−1)+(1−λ)I_(k,self,m) for slot m, where λ isthe forgetting factor of the IIR filter.

To sum up, when the power settings of the user is known e.g. by decodingcontrol channel information, the filtered self interference is removedfrom the filtered total interference in the algorithm

${{modified}\mspace{14mu}{SIR}} = {\sum\limits_{k = 1}^{L}{\frac{S_{k}}{I_{k,m} - I_{k,{self},m}}.}}$

In FIG. 5, a method for continuously updating a quality measure of aradio channel involves continuously determining a modified Signal toInterference plus noise ratio (SIR) in which the influence from selfinterference has been removed.

In detail, the method involves the steps of

determining 29 a self interference covariance matrix R_(ISI), scaled bya first fitting parameter A and continuously updated based on at leastreceived demodulated pilots and path delays in the radio channel,

determining 30 a noise covariance matrix R_(n) scaled by a secondfitting parameter B,

adapting 31 the first and second fitting parameters A, B to fit a modelof the radio channel to measured received pilots for example by using aLeast Square method, and

determining 36 the modified SIR as a relation between a signal power orthe like, the second parameter B and the noise impairment term R_(n).

The model of the radio channel is for example described by the equationR_(m)=A·R_(ISI)+B·R_(n), and is determined by estimating 28 a totalcovariance matrix R_(m) based on at least received demodulated pilots.

In accordance with the described example, the method further comprisesthe steps of determining 32 when the fitted first parameter A decreasesbelow a preset value, to thereupon set the first parameter A to zero andto re-estimate 33 the second parameter B. The second, possiblyre-estimated, parameter B is then filtered 34 with a Minimum MeanSquared Error (MMSE) based smoothing method. The possibly re-estimatedand filtered second parameter B is then post-filtered 35 in a low-passfiltering process before determining 36 the modified SIR.

In FIG. 6, an alternative method for continuously updating a qualitymeasure of a radio channel involves continuously determining a modifiedSignal to Interference plus noise ratio (SIR) in which the influencefrom self interference has been removed.

In detail, a signal power or the like is estimated 37 of pilots or acode in which the pilots are incorporated from each finger output of aRAKE receiver. Further, a noise power or the like is estimated 41 ofpilots or a code in which the pilots are incorporated from each fingeroutput of a RAKE receiver. The estimated noise power is thenpost-filtered 42 in a filter having low pass characteristics. Also, thetotal signal power or the like over all codes is estimated 38 for eachfinger output using the signal power or the like on the pilots or thecode in which the pilots are incorporated and known power settings ofthe transmitted codes. The self interference is estimated 39 for eachfinger output as the contribution of the total signal power or the likeover all codes and all finger outputs except the finger output for whichthe self interference is estimated. The estimated self interference isthen post-filtered 40 in a filter having low pass characteristics. Themodified SIR value is calculated based on the estimated signal power,post-filtered self interference estimates and post-filtered noise powerestimates.

The invention claimed is:
 1. A receiver comprising a fast power controlunit, said fast power control unit being arranged to continuouslycontrol a quality measure of a radio channel, wherein the qualitymeasure is a modified Signal to Interference plus noise ratio (SIR) inwhich an influence from self interference has been removed, wherein thefast power control unit comprises a processing unit comprising a modelof the radio channel determined at least based on known, demodulatedpilots, said model comprising a self interference impairment termR_(ISI) scaled by a first fitting parameter A and a noise impairmentterm R_(n) scaled by a second fitting parameter B, the processing unitis arranged to adapt values of the first and second fitting parametersA, B to fit the model to measured received pilots, the processing unitis arranged to determine that the radio channel has a high Carrier toInterference ratio and is a dispersive channel, thus requiringdetermination of the modified SIR; the processing unit is arranged todetermine the modified SIR as a relation between a signal power, thesecond fitting parameter B and the noise impairment term R_(n) withoutemploying the self interference impairment term R_(ISI); the processingunit is arranged to determine when the first fitting parameter Adecreases below a preset value, and thereupon set the first fittingparameter A to zero, re-estimate the value of the second fittingparameter B, and use the re-estimated second fitting parameter B indetermining the modified SIR; wherein the receiver further comprises aRAKE receiver operatively connected to the fast power control unit andarranged to feed known pilots from the respective finger outputs of theRAKE receiver to the fast power control unit; wherein the RAKE receivercomprises at least one finger output for each significant fading path inthe radio channel; and wherein the number of finger outputs of the RAKEreceiver fed to the fast power control unit is larger than the number offinger outputs fed to a RAKE combiner for data processing.
 2. A receiveraccording to claim 1, wherein the processing unit is arranged todetermine the first and second fitting parameters A, B by means of aLeast Square method.
 3. A receiver according to claim 1, wherein themodel of the radio channel is described by the equationR_(m)=A·R_(ISI)+B·R_(n), wherein the processing unit is arranged toestimate a total covariance matrix R_(m) based on at least receiveddemodulated pilots, wherein the self interference impairment termR_(ISI) is a self interference covariance matrix and wherein the noiseimpairment term R_(n) is a noise covariance matrix.
 4. A receiveraccording to claim 3, wherein the processing unit is arranged tocontinuously update the self interference covariance matrix R_(ISI)based on at least received demodulated pilots and path delays in theradio channel.
 5. A receiver according to claim 1, wherein theprocessing unit is arranged to filter a possibly re-estimated secondfitting parameter B with a Minimum Mean Squared Error based smoothingmethod and to use the filtered second fitting parameter B in determiningthe modified SIR.
 6. A receiver according to claim 1, wherein theprocessing unit is arranged to post-process a possibly re-estimatedand/or filtered second fitting parameter B in a post-filtering unitbefore determining the modified SIR, said post-filtering unit comprisinga filter having low-pass characteristics.
 7. A receiver according toclaim 1, wherein the receiver is a CDMA receiver.
 8. A receiveraccording to claim 1, wherein the fast power control unit is arranged tocompare the determined quality measure with a predetermined qualityvalue and to based on said comparison broadcast a message to atransmitter for amending the transmitting power.
 9. A receiver accordingto claim 1, wherein the receiver is embodied in a wireless communicationterminal for use in a wireless communication network.
 10. A receiveraccording to claim 1, wherein the receiver is embodied in a radio basestation for use in a wireless communication network.
 11. A receivercomprising a fast power control unit, said fast power control unit beingarranged to continuously control a quality measure of a radio channel,wherein the quality measure is a modified Signal to Interference plusnoise ratio (SIR) in which an influence from self interference has beenremoved, wherein the fast power control unit comprises a firstprocessing unit comprising a model of the radio channel determined atleast based on known, demodulated pilots, said model comprising a selfinterference impairment term R_(ISI) scaled by a first fitting parameterA and a noise impairment term R_(n) scaled by a second fitting parameterB, the first processing unit is arranged to adapt values of the firstand second fitting parameters A, B to fit the model to measured receivedpilots, the first processing unit is arranged to determine that theradio channel has a high Carrier to Interference ratio and is adispersive channel, thus requiring determination of the modified SIR;the first processing unit is arranged to determine the modified SIR as arelation between a signal power, the second fitting parameter B and thenoise impairment term R_(n) without employing the self interferenceimpairment term R_(ISI); the processing unit is arranged to determinewhen the first fitting parameter A decreases below a preset value, andthereupon set the first fitting parameter A to zero, re-estimate thevalue of the second fitting parameter B, and use the re-estimated secondfitting parameter B in determining the modified SIR; wherein thereceiver further comprises a second processing unit arranged tocalculate the self interference comprising: a power estimation unitarranged to: estimate a signal power and a noise power of the pilots ora code in which the pilots are incorporated from each RAKE fingeroutput, and to estimate the total signal power over all codes for eachfinger output using the signal power on the pilots or the code in whichthe pilots are incorporated and known power settings of the transmittedcodes, and a self interference estimating unit arranged to estimate theself interference for each finger output as the contribution of thetotal signal power over all codes and all finger outputs except thefinger output for which the self interference is estimated.
 12. Areceiver according to claim 11, wherein the receiver comprises apost-filtering unit arranged to filter the self interference estimatedin the self interference estimating unit by means of a filter havinglow-pass characteristics.
 13. A receiver according to claim 11, whereinthe receiver comprises a total interference estimating unit arranged toestimate the total interference for each finger output.
 14. A receiveraccording to claim 13 wherein the receiver comprises a post-filteringunit arranged to filter the total interference estimated in the totalinterference estimating unit by means of a filter having low-passcharacteristics.
 15. A receiver according to claim 14 wherein thereceiver comprises a SIR estimation unit arranged to estimate themodified SIR as a ratio between an estimated signal power and adifference between a possibly post-filtered self interference and apossibly post-filtered total interference.
 16. A method for continuouslycontrolling a quality measure of a radio channel, comprisingcontinuously determining a modified Signal to Interference plus noiseratio (SIR) in which an influence from self interference has beenremoved, the continuous determination of the modified SIR comprising thesteps of determining a self interference impairment term R_(ISI) scaledby a first fitting parameter A, determining a noise impairment termR_(n) scaled by a second fitting parameter B, adapting the first andsecond fitting parameters A, B to fit a model of the radio channel tomeasured received pilots, determining that the radio channel has a highCarrier to Interference ratio and is a dispersive channel, thusrequiring determination of the modified SIR; determining the modifiedSIR as a relation between a signal power, the second fitting parameter Band the noise impairment term R_(n) without employing the selfinterference impairment term R_(ISI); determining when the first fittingparameter A decreases below a preset value, and thereupon setting thefirst fitting parameter A to zero, re-estimating the second fittingparameter B, and using the re-estimated second fitting parameter B indetermining the modified SIR; using a RAKE receiver to feed known pilotsfrom respective finger outputs of the RAKE receiver to a fast powercontrol unit; wherein the RAKE receiver comprises at least one fingeroutput for each significant fading path in the radio channel; andwherein the number of finger outputs of the RAKE receiver fed to thefast power control unit is larger than the number of finger outputs fedto a RAKE combiner for data processing.
 17. A method according to claim16, wherein the first and second fitting parameters A, B are fitted bymeans of a Least Square method.
 18. A method according to claim 16,wherein the model of the radio channel is described by the equationR_(m)=A·R_(ISI)+B·R_(n), and is determined by estimating a totalcovariance matrix R, based on at least received demodulated pilots,wherein the self interference impairment term R_(ISI) is a selfinterference covariance matrix and wherein the noise impairment termR_(n) is a noise covariance matrix.
 19. A method according to claim 18,wherein the self interference covariance matrix is continuously updatedbased on at least received demodulated pilots and path delays in theradio channel.
 20. A method according to claim 19, comprising the stepof filtering a second, possibly re-estimated, fitting parameter B with aMinimum Mean Squared Error based smoothing method, wherein the filteredsecond fitting parameter B is used in determining the modified SIR. 21.A method according to claim 16, comprising the step of post-filtering apossibly re-estimated and/or filtered second fitting parameter B in alow-pass filtering process before determining the modified SIR.
 22. Themethod of claim 16, wherein said method is used for fast power control,and further comprises the steps of continuously controlling a qualitymeasure of a radio channel, comparing the determined quality measurewith a predetermined quality value, and based on said comparisonbroadcasting a message to a transmitter for amending the transmittingpower.
 23. A method for continuously controlling a quality measure of aradio channel, comprising continuously determining a modified Signal toInterference plus noise ratio (SIR) in which an influence from selfinterference has been removed, the continuous determination of themodified SIR comprising: determining a self interference impairment termR_(ISI) scaled by a first fitting parameter A, determining a noiseimpairment term R_(n) scaled by a second fitting parameter B, adaptingthe first and second fitting parameters A, B to fit a model of the radiochannel to measured received pilots, determining that the radio channelhas a high Carrier to Interference ratio and is a dispersive channel,thus requiring determination of the modified SIR; determining themodified SIR as a relation between a signal power, the second fittingparameter B and the noise impairment term R_(n) without employing theself interference impairment term R_(ISI); determining when the firstfitting parameter A decreases below a preset value, and thereuponsetting the first fitting parameter A to zero, re-estimating the secondfitting parameter B, and using the re-estimated second fitting parameterB in determining the modified SIR; estimating a signal power andestimating a noise power of pilots or a code in which the pilots areincorporated from each finger output of the RAKE receiver, estimatingthe total signal power over all codes for each finger output using thesignal power on the pilots or the code in which the pilots areincorporated and known or blind detected power settings of thetransmitted codes, and estimating the self interference for each fingeroutput as the contribution of the total signal power over all codes andall finger outputs except the finger output for which the selfinterference is estimated.